Developer Harry Gross (left) may be under the radar but his new Marriott at 1717 Broadway is over the top. The nation's tallest hotel-only building at 68 stories (right) opened over the weekend.Anne Wermiel

Meet Harry Gross, the mystery man behind the just-opened, tallest hotel-only building in the US.

Gross, a youthful-looking 73, and his G-Holdings LLC developed and own the 68-story, 1717 Broadway at West 54th Street — home to 639 Marriott-flagged guest rooms and suites. Floors 6-33, with 378 rooms, are branded as Courtyard by Marriott; floors 37-68, with 261 suites, are branded as Residence Inn by Marriott.

The tower cost an estimated $319.5 million for site acquisition, construction and “soft costs.” After years of land- and air-rights purchases and site preparation, the as-of-right project shot up in a mere 30 months.

But who’s Harry Gross, who has long shunned interviews?

“I’m originally from Brooklyn,” the polite, courtly mannered developer says. Then he adds with a laugh: “I arrived there in 1967, so I’m originally from Brooklyn.”

Israeli-born Gross made enough money in the garment business to take the real-estate plunge. He develops and owns properties without partners, a style he calls “sweat equity.”

Gross pioneered the mid-priced hotel model that’s battened on the city’s tourism and business-travel boom after 9/11. His properties are not budget brands like Sam Chang’s Comfort Inns and Hilton Gardens, but pricier, franchised Marriotts.

Beginning in the 1990s, Gross built six of them ground-up starting with Courtyard by Marriott Times Square at 114 W. 40th St. — the first “select services,” major-brand hotel of its kind at a center-city address.

But it nearly wasn’t. Gross bought the land in the 1980s, “eight or nine years earlier,” he said. “Originally, I planned to do a fashion showroom building. But literally on the way to a meeting with Citibank, I didn’t feel the time was right to develop a fashion showroom building.

“I respectfully told the bankers I did not wish to go ahead with borrowing $25 million. I paid a $250,000 penalty not to build that building — it was one of the best deals I never made,” he chuckled.

Gross started thinking about a hotel instead. His frequent business travel “focused me on a different way to approach guests. I learned what business travelers want — all the tools they need for work. They don’t want to pay for amenities they don’t use, like ballrooms.”

He approached Marriott International and its then-head Bill Marriott about a “vertical concept” limited-services hotel. “It was a first in the city,” Gross said. “All the other Courtyards were outside the center cities.

“In terms of room size, it was somewhat smaller than typical Courtyards. But with the reality of Manhattan costs, Marriott saw the need to accept what was not a brand standard.”

The West 40th Street breakthrough was followed by a Courtyard at JFK Airport in 2001 and the 45-story Residence Inn on Sixth Avenue at 39th, among others.

My colleague Lois Weiss first reported in 2001 that Gross and Marriott had struck a deal to build a much larger project at 1717 Broadway. But it would take nearly a decade to crystallize as the country’s first “double-decker” hotel — “two hotels under the same master brand but catering to different market segments,” Gross said — in a slender, glassy structure designed by Nobutaka Ashihara.

Construction of the towering twin inns was overseen by his son, Ron Gross. (Another project of what Harry calls his “small family business,” mixed-use 7 Queens Plaza North, is overseen by a second son, Etai.)

Janis Milham, senior VP of Modern Essentials and Extended Stay for Marriott International, called 1717 Broadway “a prime example of an ideal dual-branded hotel, combining two of our power brands under one roof.” Both are managed by Interstate Hotels & Resorts, which operates many Marriott properties.

Opening rates start at $300 a night for Courtyard and $350 a night for the more extended stay-oriented Residence Inn.

A single lobby services both hotels. Colorful, graffiti-inspired art by William DeBilzan brings a hip touch to sleek public spaces and hallways. New Year’s Eve guests on higher floors will be able to watch the Times Square ball drop from a rare vantage point — above.

The twins arrive in the midst of a tourism and business-travel boom.

Lodging Advisors’ analyst/consultant Sean Hennessey reports average Manhattan occupancy for 2013 at a national-high 87 percent and average daily room rate of $290 — and forecasts the same occupancy for 2014 and rates rising to $306.

Gross isn’t quite finished with 1717 Broadway. There’s no restaurant yet — but 9,000 square feet of retail space on the second floor, to be leased, could be used for one. A fifth-floor bar will open soon.

There’s also a 3,000 square-foot retail space at street level, small but precious in the thriving Broadway corridor that links Columbus Circle and Times Square.

Although blue lights already define the tower’s three setbacks, more dramatic illumination is to come.

Strips of LED lighting allow programming effects by artists that will be visible from afar.

It will be a new skyline ornament in an under-lit part of Midtown. But “that’s what keeps me kicking,” Gross says with a smile — “the new.”